In an electronic device such as a mobile phone, in order to mount a function of noncontact short distance communication, an antenna module for RFID (Radio Frequency Identification) is used.
The antenna module communicates with an antenna coil mounted on a transmitter such as a reader-writer by using electromagnetic coupling. More specifically, the antenna module can drive an IC functioning a communication processing unit that causes the antenna coil to receive a magnetic field from the reader-writer to convert the magnetic field into electric power.
In the antenna module, magnetic fluxes having a certain value or more and transmitted from a reader-writer must be received by an antenna coil to reliably perform communication. In a conventional antenna module, a loop coil is arranged in the housing of a mobile phone, and the loop coil receives magnetic fluxes from a reader-writer.
However, in an antenna module incorporated in an electronic device such as a mobile phone, since a metal of a circuit board in the device or a battery pack reflects a magnetic fluxes from a reader-writer due to an eddy current generated by receiving a magnetic field from the reader-writer, the magnetic fluxes reaching the loop coil decrease. Since the magnetic fluxes reaching the loop coil decrease, the antenna module requires a loop coil having a certain opening area to collect required magnetic fluxes. Furthermore, the magnetic fluxes must be increased by using a magnetic sheet.
As described above, although magnetic fluxes from the reader-writer are reflected by an eddy current flowing in the circuit board of an electronic device such as a mobile phone, a component of a magnetic field along a plane direction of the circuit board is present on a housing surface of the electronic device. A technique in which a coil is caused to function as antenna by receiving the component is described in Patent Document 1. More specifically, in Patent Document 1, an antenna structure obtained by winding a coil on a ferrite core to reduce an occupied area of a coil is described.